The possibility to determine the position of a mobile device has enabled application developers and wireless network operators to provide location based, and location aware, services. Some examples of such services are guiding systems, shopping assistance, friend finder, presence services, community and communication services and other information services giving the mobile user information about their surroundings.
In addition to commercial services, governments of many countries require network operators to determine the position of an emergency call. For instance, the governmental requirements in the United States (FCC E911) mandate that it must be possible to determine the position of a certain percentage of all emergency calls. The requirements make no difference between indoor and outdoor environment.
Positioning methods that use the wireless network can be grouped in two main groups. The first group includes methods that are based on the radio cell to which a mobile terminal is attached, e.g. by using Cell-ID or a combination of cell-ID and timing advance (TA) measurements. The TA measurement principle is depicted in FIG. 1. Briefly, the travel time of radio waves from the eNodeB (enhanced NodeB) to the UE (user equipment) and back is measured. The distance from eNodeB to UE is given by
  r  =      c    ⁢          TA      2      where TA is the timing advance and where c is the speed of light.
The TA alone defines a circle, or if the inaccuracy is accounted for, a circular strip around the eNodeB. By combining this information with the cell polygon, left and right angles of the circular strip can be computed. The terminal position is determined as the intersection of the serving cell and the circular strip.
As for other terrestrial positioning methods such as observed time difference of arrival (OTDOA), these suffer from too low detection performance to provide acceptable performance, at least in the basic configuration.
Another approach is provided by so called fingerprinting positioning. Fingerprinting positioning algorithms operate by creating a radio fingerprint for each point of a fine coordinate grid that covers the Radio Access Network (RAN). The fingerprint may e.g. include: the cell IDs that are detected by the terminal, in each grid point; quantized path loss or signal strength measurements, with respect to multiple eNodeBs, performed by the terminal, in each grid point (an associated ID of the eNodeB may also be needed); quantized TA, in each grid point (an associated ID of the eNodeB may also be needed); and/or quantized AoA information.
Whenever a position request arrives to the positioning method, a radio fingerprint is first measured, after which the corresponding grid point is looked up and reported. This requires that the point is unique. The database of fingerprinted positions can be generated in several ways. A first alternative is to perform an extensive surveying operation that performs fingerprinting radio measurements repeatedly for all coordinate grid points of the RAN (radio access network). The disadvantages of this approach include: the surveying required becomes substantial also for small cellular networks; and the radio fingerprints are in some instants (e.g. signal strength and pathloss) sensitive to the orientation of the terminal, which is particularly troublesome for handheld terminals. For fine grids, the accuracies of the fingerprinted positions therefore become highly uncertain. This is unfortunately seldom reflected in the accuracy of the reported geographical result.
Another approach, applied e.g. in Adaptive Enhanced Cell IDentity positioning (AECID), is to replace the fine grid by high precision position measurements of opportunity, and to provide fingerprinting radio measurements for the points. This avoids the above drawbacks. However, algorithms for clustering of high precision position measurements of opportunity must be defined and algorithms for computation of geographical descriptions of the clusters must also be defined.
With regard to AoA, the AoA measurement standardized for LTE (long term evolution) is defined as the estimated angle of a UE with respect to a reference direction which is the geographical north, positive in the clockwise direction. AoA can reduce the angular uncertainty as compared to cell ID and TA positioning if combined with TA, as illustrated in FIG. 2.
Future wireless systems like LTE and WiMAX utilize so called Multiple-Input Multiple-Output (MIMO) transmission schemes to increase spectral efficiency. MIMO schemes assume that the transmitter and receiver are both equipped with multiple antennas, and that multiple modulated and precoded signals are transmitted on the same time-frequency resource element e.g. as illustrated in FIGS. 3-4.
Mathematically the transmitted signal for a particular frequency/time resource element (k,l) can be expressed as:x(k,l)=WRI,PMI(k,l)·s(k,l)  (1)where s(k,l) is a vector with elements si, i=1, . . . , RI, and where si is a modulated symbol, RI is the rank indicator, i.e. the number of signals (layers) transmitted on the same time-frequency resource element, WRI,PMI(k,l) is the so-called precoding matrix of dimension Ntx×RI, x(k,l) is a vector of transmitted signals, where x(i), i=1, . . . , Ntx, is the signal transmitted from the ith transmit antenna. The indices RI and PMI attached to the precoder indicate that the precoder is selected from a finite set of precoders as will be explained later herein. The signal is transmitted over a channel with channel matrix H which is of dimension Nrx×Ntx. The received signal vector is then an Nrx dimensional vector given by:y=Hx=HWRI,PMIs+e  (2)where e is the noise and interference vector, with covariance matrix Re. In equation (2) and further in the description, indices (k,l) have been omitted to simplify the notation.
The UE can estimate the channel matrix H based on reference symbols transmitted from all transmit antennas. The reference symbols are typically transmitted on orthogonal resources, i.e. a resource element used for transmitting reference symbols from one antenna is not used by any other antenna. An example of such signals are cell-specific reference signals (CRS). FIG. 4 illustrates mapping of the LTE CRS onto the time/frequency grid for CRS transmitted from one (the top-row illustration), two (second row of illustrations) and four antenna ports (bottom-row illustrations).
The used precoder matrix is signalled to the UE so that the UE can form the composite channel HW and demodulate the signal s. The precoder matrix W is selected from a codebook with a finite set of elements. For two TX (transmit) antennas the codebook in Table 1 is used.
TABLE 1Codebook for transmission on antenna ports {0, 1}[3GPP TS 36.211].Precoder matrixTransmission rank indicator (RI)index (PMI)120      1          2        ⁡      [                            1                                      1                      ]        1          2        ⁡      [                            1                          0                                      0                          1                      ]   1      1          2        ⁡      [                            1                                                  -            1                                ]        1    2    ⁡      [                            1                          1                                      1                                      -            1                                ]   2      1          2        ⁡      [                            1                                      j                      ]        1    2    ⁡      [                            1                          1                                      j                                      -            j                                ]   3      1          2        ⁡      [                            1                                                  -            j                                ]  —
The choice of precoder matrix includes both rank selection (number of layers) and codebook index selection. For Ns=1, the codebook contains 4 alternative precoders. For rank 2, the codebook contains 3 alternatives. Assume now that the precoder selection is restricted to rank 1 elements. Then an estimate of the symbol s can be computed by combining the received signal in the following way (so-called IRC or MMSE weighting):ŝ=(HWRI,PMI)HRe−1y=(HWRI,PMI)HRe−1(HWRI,PMI)s+(HWRI,PMI)HRe−1e  (3)The SNR after signal combining can be written:SNR=(HWRI,PMI)HRe−1(HWRI,PMI)  (4)
To maximize the throughput, it is desired to choose the RI and PMI so that SNR is maximized. This requires knowledge of the channel matrix H and the covariance Re, knowledge which is only present in the UE. Therefore the standards contain mechanisms so that UE can report the preferred RI and PMI to the base station.
Described next is how precoder selection relates to the location of the UE, based on the geometry shown in FIG. 5. Rank 1 transmission is assumed so that the precoder W is a column vector with kth element equal to wk. The signal sk(t)=wk·s·exp(iωct) is transmitted from the kth antenna. The received noise-free signal is rk(t)=h·wk·s·exp(iωc(t−τk)) where h is the attenuation of the signal, τk is the delay from base station antenna k to the UE. This is τk=dk/c where c is the speed of light. Assuming UE is far from base station relative to the distance between antennas, the relative delay can be written as τk−τj=Δk,j sin(θ)/c. Assuming arbitrarily the sensor j=1 is the reference, then:rk(t)=h·wk·s·exp(−iωcτ1)·exp(iωc(t−(τk−τ1)))=h·wk·s·exp(−iωcτ1)exp(iωct)·exp(iωcΔk,1 sin(θ)/c)=hc·wk·exp(iωct)·exp(−i2πΔk,l sin(θ)/λ)·s Here λ is the wavelength and we have used the notation hc=h·exp(−ωcτ1).
The resulting SNR assuming for simplicity that Re=I, h=1 is SNR=(w1+ . . . +wNexp(i2π{tilde over (Δ)}N,1 sin(θ))H (w1+ . . . +wNexp(i2π{tilde over (Δ)}N,1 sin(θ)). The SNR for different precoders as a function of the angle of arrival (θ) is plotted in FIG. 6 for two TX antennas and in FIG. 7 for four TX antennas with {tilde over (Δ)}k,l=2*(k−1). There is a strong relation between azimuth and the index of the precoder maximizing SNR. Not all precoders may be useful. In the four TX case, only five candidate precoders are needed.
For two TX antennas it can be seen that 3 different precoders provide maximum SNR in different azimuth angles. Therefore a measurement that indicates any of these precoders can be viewed as a direction measurement. The fourth precoder has little use in this application. Therefore the measurement can be restricted to only the three candidates having a distinct lobe.
Another possibility is to make several measurements in which the strongest precoder from the first measurement is excluded. Thereby it may be possible to reduce the angular extension further. For example, if the first measurement indicates that the position is in the main lobe around θ=0 degrees, the a second measurement excluding that precoder may provide additional information about whether the position is at a negative or positive angle relative to θ=0. Thereby the angular extension can be reduced by 50%.
The description for AoA determination with precoding indices has been given so far for DL (downlink). Precoding matrix indicator (PMI) and rank indicator (RI) reporting currently defined by the standard is for the serving cell only.
PMI and RI in LTE may be reported for the serving cell by the UE to facilitate link adaptation and channel dependent codebook based precoding in the DL. The time and frequency resources that can be used by the UE to report PMI and RI are controlled by the eNodeB. PMI and RI reporting can be periodic or aperiodic. Periodic PMI and RI reports can be transmitted on PUCCH (UMTS LTE Physical Uplink Control Channel) or PUSCH (UMTS LTE Physical Uplink Shared Channel). Aperiodic PMI and RI reports can be transmitted on PUSCH. The minimum reporting interval for PMI and RI is 1 ms. PMI for neighbor cells of the UE, however, cannot be measured or reported with conventional methods. Precoders are also defined for UL (uplink), but the information is only available in the radio nodes (eNodeBs).
Three key network elements in an LTE positioning architecture are the LCS client, the LCS target and the LCS Server. The LCS Server is a physical or logical entity managing positioning for a LCS target device by collecting measurements and other location information, assisting the terminal in measurements when necessary, and estimating the LCS target location. An LCS Client is a software and/or hardware entity that interacts with a LCS Server for the purpose of obtaining location information for one or more LCS targets, i.e. the entities being positioned. LCS Clients may reside in the LCS targets themselves. An LCS Client sends a request to LCS Server to obtain location information, and the LCS Server processes and serves the received requests and sends the positioning result and optionally a velocity estimate to the LCS Client. A positioning request can be originated from the terminal or the network.
Two positioning protocols operating via the radio network exist in LTE, LPP and LPPa. LPP is a point-to-point protocol between an LCS Server and an LCS target device, used in order to position the target device. LPP can be used both in the user and control plane, and multiple LPP procedures are allowed in series and/or in parallel thereby reducing latency. LPPa is a protocol between an eNodeB and an LCS server specified only for control-plane positioning procedures, although it still can assist user-plane positioning by querying eNodeBs for information and eNodeB measurements. The SUPL protocol may be used as a transport for LPP in the user plane. In the user plane with SUPL, a UE is typically referred to as SUPL Enabled Terminal (SET) and the LCS platform is typically referred to as SUPL Location Platform (SLP). An LPP extension LPPe is also defined by OMA and may be used to extend the LPP signalling, e.g. to provide more extended position reports or provide more assistance data, e.g., to better support measurement of a certain method or to support more methods and RATs. Other extensions may potentially be supported by LPP in the future.
A high-level architecture defined in the current standard is illustrated in FIG. 8, where the LCS target is a terminal, and the LCS Server is an E-SMLC (evolved serving mobile location center) or an SLP (SUPL location platform). In FIG. 8, the control plane positioning protocols with E-SMLC as the terminating point are labelled LPP, LPPa and LCS-AP and the user plane positioning protocol is labelled SUPL/LPP. SLP may include two components, SPC and SLC, which may also reside in different nodes. In an example implementation, SPC has a proprietary interface with E-SMLC, and Llp interface with SLC, and the SLC part of SLP communicates with P-GW (PDN-Gateway) and the external LCS client.
Additional positioning architecture elements may also be deployed to further enhance performance of specific positioning methods. For example, deploying radio beacons is a cost-efficient solution which may significantly improve positioning performance indoors and also outdoors by allowing more accurate positioning, for example, with proximity location techniques.
UE measurements for UE-assisted positioning are transmitted over LPP and/or SUPL. The current standard does not define a UE AoA measurement, and the measurement cannot be currently signalled over LPP to the positioning node.
The current standard defines AoA measurements only for eNodeBs, which can be reported by the radio node to the positioning node, and there is no requirement on the measurement accuracy. The AoA measured at the eNodeB is defined as the estimated angle of a user with respect to a reference direction, where the reference direction is the geographical North, positive in a counter-clockwise direction. The AoA measured at eNodeB is determined at the eNB antenna for an UL channel corresponding to the UE. The positioning result may be signalled between: the LCS target and LCS server, e.g. over LPP protocol; the positioning servers (e.g., E-SMLC and SLP), over standardized or proprietary interfaces; the positioning server and other network nodes (e.g., E-SMLC and MME/MSC/GMLC/O&M/SON); and the positioning node and LCS Client (e.g., between E-SMLC and PSAP or between SLP and External LCS Client).
Several problems have been identified with conventional techniques. PMI can be used to enable AoA measurements in the UE, but PMI measurements performed in DL and the corresponding signalling are limited to the serving cell only. Using PMI for AoA in UL has not been considered. Furthermore, precoding estimation in UL is limited to the serving cell also and the information is only available in radio nodes (e.g. eNodeBs). No UE AoA measurement is defined in the standard, and such no possibility to report it to the eNodeB or positioning node. There is no signalling means for signalling of PMI to the positioning node. Interference cancellation (IC) is not currently used for enhancing PMI-based AoA, in DL or UL. IC is not currently used for enhancing multi-cell AoA measurements, in DL or UL. Due to large false alarms that occur without IC, accuracy of AoA positioning, AoA+TA positioning, AECID and other fingerprinting positioning using AoA is relatively poor.